The mechanical response of engineering materials evaluated through continuum fracture mechanics typically assumed that a crack or void initially exists, but it did not provide information about the nucleation of such flaws in an otherwise flawless microstructure. How such flaws originate, particularly at grain (or phase) boundaries was less clear. Experimentally, “good” vs. “bad” grain boundaries were often invoked as the reasons for critical damage nucleation, but without any quantification. The state of knowledge about deformation at or near grain boundaries, including slip transfer and heterogeneous deformation, was reviewed to show that little work was done to examine how slip interactions could lead to damage nucleation. A fracture initiation parameter developed recently for a low ductility model material with limited slip systems provided a new definition of grain boundary character based upon operating slip and twin systems (rather than an interfacial energy based definition). This provided a way to predict damage nucleation density on a physical and local (rather than a statistical) basis. The parameter assesses the way that highly activated twin systems were aligned with principal stresses and slip system Burgers vectors. A crystal plasticity-finite element method based model of an extensively characterized microstructural region was used to determine if the stress–strain history provided any additional insights about the relationship between shear and damage nucleation. This analysis shows that a combination of a crystal plasticity-finite element method model augmented with the fracture initiation parameter shows promise for becoming a predictive tool for identifying damage-prone boundaries.

The Role of Heterogeneous Deformation on Damage Nucleation at Grain Boundaries in Single Phase Metals. T.R.Bieler, P.Eisenlohr, F.Roters, D.Kumar, D.E.Mason, M.A.Crimp, D.Raabe: International Journal of Plasticity, 2009, 25[9], 1655-83